A liquid crystal display device is provided with a backlight unit for irradiating a liquid crystal panel (LCD) with light from its rear side and illuminating the liquid crystal panel. The backlight unit includes an illuminating light source, a light guide plate that diffuses light emitted from the light source to irradiate the liquid crystal panel with the light, a prism sheet or diffusion sheet for equalizing the light emitted from the light guide plate and other such components.
A known example of such backlight unit is disclosed in JP 9-304623 A.
FIG. 20 is a schematic sectional view of an area light source device disclosed in JP 9-304623 A.
The area light source device (backlight unit) shown in FIG. 20 is formed by embedding fluorescent lamps 102 in a light guide plate 100, then arranging a reflective sheet 104 on a rear surface of the light guide plate 100 and laminating a transmitted light amount correction sheet 106, a light diffuser 108, and a prism sheet 110 on a light exit surface of the light guide plate 100.
The light guide plate 100 has a substantially rectangular shape and is made of a resin into which fine particles functioning to diffuse illumination light are dispersed. Also, the light guide plate 100 has a flat upper surface, which is used as the light exit surface. Further, grooves 100a each having a U shape in section are formed in the rear surface (surface opposite to the light exit surface) of the light guide plate 100. The fluorescent lamps 102 are fitted into their respective grooves. A light amount correction surface 100b for promoting the emission of illumination light is formed on the light exit surface of the light guide plate 100, except in portions just above the fluorescent lamps 102.
As mentioned above, JP 9-304623 A describes that, the fine particles are mixed into the resin to form the light guide plate 100 and in addition, the illumination light is promoted to exit by the light amount correction surface 100b formed on a part or all of the light exit surface except in the portions just above the fluorescent lamps 102, making it possible to reduce the total thickness and the unnatural unevenness in luminance of the light emitted from the light guide plate 100.
For the purpose of obtaining a backlight for a liquid crystal display device, which enables reduction in size, weight, and thickness of the liquid crystal display device as well as reduction in cost and power consumption without reducing the illumination amount of the backlight, JP 8-62426 A discloses a light guide plate having a rectangular illumination surface, a groove rectangular in section which is gouged out at a central portion with respect to the short sides in parallel with the long sides and in which a light source is inserted, and a rear surface formed such that a plate thickness is gradually reduced from the groove toward both lateral surfaces on the long sides.
In addition, for the purpose of obtaining a backlight unit that enables a thin frame and reduction of the total thickness of a liquid crystal display device and which is bright with high use efficiency, JP 10-133027 A discloses a light guide member (light guide plate) having a parabolic shape in section along the width direction of a concave portion where the light source is arranged, with the depth direction being a major axis.
In backlight units using those light guide plates, luminance distributions involving bright and dark lines are produced, so a variety of methods have been proposed for improving them (see, for example, JP 5-127156 A, JP 6-235825 A, and JP 2001-42327 A).
JP 5-127156 A describes a liquid crystal display device having a dotted area printed on a surface of a diffuser plate in order to block light transmission. The reference also states that by making the printed dots dense in the region where a cold cathode fluorescent lamp is positioned right beneath while making them less dense with the increasing distance from that region, whereby the quantity of light emitted toward the diffuser plate becomes uniform as it reaches every part of the plate.
JP 6-235825 A describes a light guide plate in which a light scattering layer is formed on the underside of the plate member in such a way that its area percentage increases with the increasing distance from a linear light source. The reference states that the light scattering layer is used to ensure that the light within the plate member of the light guide plate is picked up from the topside in response to a change in luminance from the linear light source.
JP 2001-42327 A describes a light guide plate having a transmission adjusting means which is formed as a density-modulated dot pattern that is printed in highly reflective ink to provide a strip in a longitudinal direction of a transparent substrate in such a way that the increase in luminance due to direct incidence of light from a linear light source is suppressed by inversely counteracting the quantity of that light, said density-modulated dot pattern being formed by decreasing the dot area or the number of dots from the lateral side of said projecting edge inward of the light exit plane.
As a result, the light from the linear light source is reflected by each projecting edge to reach the transmission adjusting means formed on the light emerging plane of its surface, whereupon the density modulation by that adjusting means adjusts the amount of transmission and that of anti-transmission in such a way that they both increase gradually from the lateral side of each projecting edge inward of the light exit plane, so that the increase in luminance that occurs at the area where the linear light source is provided on account of direct incidence of the light from the linear light source is suppressed to ensure uniformity in the irradiation luminance of the light guided by said light guiding means.
JP 2004-170698 A describes a light diffusing sheet as an optical member that is to be used in a backlight unit for a liquid crystal display device, the sheet having printed thereon a luminance distribution reversed image which is formed by gradation reversal of the data obtained by measuring at least the luminance distribution on the light exit surface of the backlight unit.
The luminance distribution reversed image has a gradation pattern of high precision that reflects the luminance distribution on the light exit surface of the backlight unit, so the light that has passed through the light diffusing sheet after emerging from the light guide plate has such a small degree of unevenness in luminance that it is free from the occurrence of bright lines.